The present invention relates to reactors for the processing of materials in or carried by a gaseous phase by means of corona discharges.
Considerable effort is being expended on the development of techniques for carrying out processes in the gaseous phase, using species which have been activated by corona discharges. Corona discharges occur in gaseous media when the localised electric field in the neighbourhood of a body exceeds the electrical breakdown voltage of the gaseous medium. Some existing corona discharge reactors consist of a chamber having an inlet and an outlet for a gaseous medium, an axial inner electrode and a cylindrical outer electrode surrounding the inner electrode. The electrical discharge within such reactors consists of streamers extending from the inner electrode towards the outer electrode. For the effective processing of a gaseous medium passing through the reactor, it is necessary to produce as many corona streamers as possible because any space which does not contain such streamers is dead space as far as the processing of the gaseous medium is concerned.
In one type of known corona discharge reactor the central electrode is in the form of a wire. However, the rapid fall-off in the electric field in a central wire electrode type of corona discharge reactor means that the distribution of streamers is very sensitive to wire position. This phenomenon limits the efficiency with which gaseous medium passing through the reactor can be treated.
Existing central wire electrode corona discharge reactors have diameters of a few centimetres and lengths of a few tens of centimeters. To process reasonable volumes of gas, high flow rates are required, which in turn tend to increase the mechanical instability of the central electrode due to aerodynamic effects as well as leading to short residence times in the reactor chamber of the medium to be processed. Hence, existing corona discharge reactors are limited inherently in their effectiveness.
Other corona discharge reactors, see for example our earlier patent GB 2 282 738, U.S. Pat. No. 5,041,145, U.S. Pat. No. 5,268,151 or U.S. Pat. No. 4,966,666, make use of central electrodes which have a larger diameter. Among other things this reduces the electric field gradient in the region of the central electrode at the same voltage, but there still remains a limitation on the separation between the inner and outer electrodes if an effective corona discharge is to be maintained. Merely increasing the length of corona discharge reactors does not provide an answer to the problem because the gas flow resistance becomes excessive. Also, corona discharge reactors operate in a pulsed manner, and the time taken for an energising pulse to traverse the length of the central electrode, provides another factor which limits the length of a corona discharge reactor.
GB specification 2 008 369A discloses an ozone generator which includes a,plurality of parallel electric discharge chambers each of which has a central wire electrode. The wires are supported at their ends by two grid structures to one of which a common feed wire is connected to which, in use a d.c. voltage is applied.
As the device is operated in a d.c. mode, no a.c. current distribution effects have to be considered.
GB patent 1 589 394 discloses an ozone generator which includes a number of parallel corona discharge chambers, a single power supply, which may produce pulsed d.c., a.c., or a mixture of both, potentials is used, but no attempt is made to equalise the distribution of the power supplied to the discharge chambers, either in terms of magnitude or time.
U.S. Pat. No. 4,495,043 discloses an ozoniser in which there is a plurality of ozone producing chambers which are connected to a single pulsed a.c. power supply. However, the ozone producing chambers are not operated simultaneously or, continuously, but are operated sequentially in a pulsed a.c. mode, the duration of each pulse being related to the passage time of a pulse of ozone-producing gas through the corresponding ozone producing chamber, and the intervals between the pulses: applied to a given ozone-producing chamber being such that ozone produced by one power pulse is cleared from the chamber before the next power pulse is applied to that ozone producing chamber.
The patent is concerned mainly with the design of the power supply. The question of the electrical relationship between the ozone-producing chambers is not addressed at all.
U.S. Pat. No. 5,009,858 discloses an ozoniser in which ozone is produced by a silent electric discharge in a number of chambers operated in parallel from a common power supply. The electrical relationship between one ozone producing chamber and another is not discussed at all, but it would appear that the ozone producing chambers are operated in a continuous a.c. mode.
WO 99/15267 discloses a corona discharge reactor for processing a gaseous medium. The gaseous medium is fed via inlet and outlet plenum chambers through an array of reactor chambers. Each reactor chamber has an inner axial electrode and is formed as a longitudinal, circular section, gas passage through an electrode assembly, which thus provides each reactor chamber with an outer electrode concentric with its inner electrode. Pulsed electrical power is applied simultaneously or sequentially to the inner electrodes of each of the reactor chambers. There is, however, no disclosure relating to balancing the flow of gaseous medium through the reactor chambers.
It is an object of the present invention to provide an improved form of corona discharge reactor for use in the processing of gaseous media by means of electrically activated species.
The term gaseous medium includes the case when one or more constituents of the medium is in the form of an aerosol, or finely divided solid matter carried by a gaseous phase.
According to the present invention there is provided a modular corona discharge reactor for the processing of gaseous media, comprising an inlet duct and an outlet duct, a plurality of vertically mounted corona discharge reactor chambers positioned between the inlet and outlet ducts and communicating therewith, each corona discharge reactor having a cylindrical outer electrode structure and an axial inner electrode structure, a high voltage pulse power supply positioned in close coupled vertical alignment with the inner electrode of the reactor chamber and connected directly thereto, each high voltage pulse power supply being adapted to produce voltage pulses of a magnitude sufficient to excite a corona discharge in a gaseous medium to be processed as it passes through the associated reactor chamber, means for balancing the flow through the reactor chambers of the gaseous medium to be processed, a low voltage power supply connected to each high voltage pulse power supply and a control unit adapted to control the action of the low voltage power supply to cause the high voltage pulse power supplies to apply high voltage pulses to the corona discharge reactor chambers with a pre-determined temporal relationship.
In a particular embodiment of the invention, the reactors are arranged into groups of two reactors in series between the inlet and outlet ducts and the means for balancing the flow through the reactor chambers of the gaseous medium to be processed comprises a flow control device situated between the reactor chambers of each pair of reactor chambers.
A simple form of flow control device is a calibrated orifice plate.
As corona discharge reactors are noisy devices, electromagnetically, preferably at least the corona discharge reactor chambers are provided with electromagnetic screening.